Interpretive Summary: Tea is the most popular beverage worldwide. Recent studies indicate that tea has a wide range of effects on human health. It has been reported that tea reduces cancer risk in humans, reduces carcinogen-induced malignancies in animals, inhibits tumor formation and tumor growth, and is a potential neuroprotective agent in Alzheimer’s and Parkinson’s diseases and diabetes. A number of studies have indicated that extracts of green tea have insulin-like activity. We hypothesized that the anti-diabetic effects of green tea involve regulation of gene expression for glucose uptake and insulin signaling. In this study, we explored the relationship between consumption of a green tea extract and the messenger ribonucleic acid (mRNA, whose sequence determines protein sequence) levels of factors that determine glucose uptake and participate in the insulin signaling pathway in the liver and muscle of rats fed a high sugar diet known to induce insulin resistance and oxidative stress. We demonstrated that the extract of green tea regulated gene expression in the glucose uptake and insulin signaling pathways in rats fed a fructose-rich diet. These results suggest that drinking equivalent amounts of green tea may play a role in the prevention of insulin resistance and diabetes in individuals consuming a high sugar (fructose) diet.

Technical Abstract:
Green tea has anti-diabetic and anti-obesity activities, but the molecular mechanisms of these effects have not been fully understood. Quantitative real-time PCR was used to investigate the relative expression levels and the effects of a polyphenol extract green tea (1 and 2 g solid extract/kg diet) on the expression of glucose transporter family genes (Glut1/Slc2a1, Glut2/Slc2a2, Glut3/Slc2a3, Glut4/Slc2a4) and insulin signaling pathway genes (Ins1, Ins2, Insr, Irs1, Irs2, Akt1, Grb2, Igf1, Igf2, Igf1r, Igf2r, Gsk3b, Gys1, Pik3cb, Pik3r1, Shc1,and Sos1) in liver and muscle of rats fed a high-fructose diet known to induce insulin resistance and oxidative stress. Glut2 and Glut4 were the major Glut mRNAs in rat liver and muscle, respectively. Green tea polyphenol extract (1 g) increased Glut1, Glut4, Gsk3b, and Irs2 mRNA levels by 110%, 160%, 30%, and 60% in the liver, respectively, and increased Irs1 by 80% in the muscle. Green tea polyphenol extract (2 g) increased Glut4, Gsk3b, and Pik3cb mRNA levels by 100%, 30%, and 30%, but decreased Shc1 by 50% in the liver and increased Glut2, Glut4, Shc1, and Sos1 by 60%, 80%, 50%, and 50% in the muscle. This study shows that a concentrated extract of green tea regulates gene expression in glucose uptake and insulin, signaling pathway in rats fed a fructose-rich diet.